As is known, baghouses are used extensively in power generating stations and the like for air pollution control. In a baghouse of this type, dust-laden air enters a plenum chamber and then flows upwardly through apertures in an upper wall of the plenum chamber and into a plurality of vertically-elongated tubular filter bags which separate the dust from the air as it flows through the fabric walls of the bags. The bottom of each filter bag is open and is secured to the periphery of a thimble which extends upwardly from the upper wall of the plenum chamber and surrounds an associated one of the aforesaid apertures. In the usual case, the bottom of the thimble is essentially flush with the upper wall of the plenum chamber.
When gas in the plenum chamber exits through the aperture and the thimble into the filter bag, part of the gas approaches along the center line of the exit path through the aperture; however another part comes toward the aperture radially along the underside of the upper wall of the plenum from all directions. The part approaching along the underside of the upper wall has momentum toward the center of the aperture with the result that this portion of the entering air may not be fully turned into alignment with the bag axis. As a result, it will strike the fabric of the filter bag at a greater angle of incidence. Particles carried by the gas stream will then strike a grazing blow on the fabric and increase wear rate.
The problem of bag wear due to the incidence of air entering the thimble in a sideways or radial direction can be alleviated by extending the thimble beneath the upper wall of the plenum chamber in order that the entire cross section of the airstream, when it intersects the bottom of the bag, will be essentially parallel to the longitudinal axis of the bag. In the past, thimble extensions of this type have normally comprised an integral part of the thimble itself; and, consequently, were formed from sheet metal of the same gage or thickness as the main portion of the thimble which must be heavy enough to withstand the pull of the bag, handling stresses and welding requirements.